Adding a new card is simple. Click on the Deposit option. Here you can find next to the amount 'Change'. Click here following the next steps and you will be able to enter you new card details.
If for any reason you have an error in this section, it could be that you have too many cards registered, so just contact our customer service and they can take care of the rest for you.
If you would like to remove an old saved card, please go to this article for more information.